The present invention relates, in general, to electronic devices and, more particularly, to variable capacitors.
A tuner comprised of an inductor (L) and a variable capacitor (C) is widely used to provide a variable resonant frequency. Conventionally, a voltage controlled tuner includes a varactor serving as the variable capacitor. A varactor is a reverse biased diode. As the voltage applied to the varactor varies, the depletion width of in the diode changes, and so do the capacitance of the varactor and the resonant frequency of the voltage controlled tuner.
However, the range of capacitance variation of a varactor is quite limited. For example, the capacitance of a varactor usually varies approximately by a factor of two over a voltage range of approximately four volts. In low voltage applications such as portable wireless communication, a capacitance variation by a factor of five over a voltage range of approximately one volt is often desirable. Further, the diode in a varactor conducts when forward biased, which presents a leakage problem for the varactor.
Accordingly, it would be advantageous to have a voltage variable capacitor and a method for fabricating the capacitor. It is desirable for the capacitor to have a high quality factor and a low leakage. It is also desirable for the capacitor to have a large capacitance variation range over a small voltage range. It is further desirable for the capacitor to be fabricated with other integrated circuit devices in a monolithic semiconductor chip. It would be of further advantage for the method to be compatible with existing semiconductor integrated circuit fabrication processes.